Improved sensors are needed that can operate in harsh environments for the next generation of technologies for higher efficiency, lower emission fossil-fueled power plants including oxy-fuel combustion processes for carbon capture and sequestration and coal gasification to produce syngas which can be converted to electrical power using solid-oxide fuel cells or gas turbines. Improved harsh environment sensors and controls would also enable significant gains in energy efficiency for the existing fleet of coal-fired power plants and a number of major domestic manufacturing industries. In particular, chemical sensors capable of operating at elevated temperatures in highly reducing, oxidizing, and/or corrosive environments can be leveraged across a broad range of applications including coal gasification, combustion turbines, solid oxide fuel cells, and advanced boiler systems.
Optical sensors are of increasing interest for a wide range of embedded sensing applications due to a number of inherent advantages as compared to other sensor technologies including the ability to monitor several different optical properties of a selected sensing material (transmission, reflection, luminescence). However, an indirect relationship between electrical and optical properties of most metal oxide based films implies that the large body of existing work on semiconducting materials for chemi-resistive based gas sensing applications provides only limited guidance for development of sensor materials for optical sensing applications. Material systems with useful optical responses specifically tailored for the application of interest will therefore be required.
Metal oxides such as WO3 have been utilized as optical sensors for H2 while other metal oxides such as NiO and Co3O4 have been explored for optical sensing of reducing gases such as CO. However, these materials suffer from limited temperature stability in highly reducing conditions and typical dynamic ranges of measured output signals based on absorbance or reflectance have limited their practical use in a gas sensing, instrument. See e.g. Ando, “Recent advances in optochemical sensors for the detection of H2, O2, O3, CO, CO, and H2O in air,” Trends in Analytical Chemistry 25(10) (2006); see also Korotcenkov, “Metal oxides for solid-state gas sensors: What determines our choice?” Materials Science and Engineering B 139 (2007). Incorporation of noble metals such as gold nanoparticles into these metal oxides has generally been employed to enable responses that are suitable for practical gas sensing. See e.g., Schleunitz et al., “Optical gas sensitivity of a metal oxide multilayer system with gold-nano-clusters,”Sensors and Actuators B 127 (2007); see also Gaspera et al., “CO optical sensing properties of nanocrystalline ZnO—Au films: Effect of doping with transition metal ions,” Sensors and Actuators B 161 (2012); see also Gaspera et al., “Enhanced optical and electrical gas sensing response of sol-gel based NiO—Au and ZnO—Au nanostructured thin films,” Sensors and Actuators B 164 (2012); and see Ando et al., “Combined effects of small gold particles on the optical gas sensing by transition metal oxide films,” Catalysis Today 36 (1997). In other cases, metal oxides such as ZnO with various dopants have been utilized and absorbance changes have been noted for gases such as ammonia, methanol and ethanol, however the mechanism has generally been attributed to the adsorption of oxygen molecules at the metal oxide surface and the dopant was utilized to enhance catalytic activity, and correspondingly measurement temperatures have been limited to below about 100° C. The time constants for the measured responses also tend to be prohibitively long such that they are not practical for a gas sensing device. See e.g., Renganathan et al., “Gas sensing properties of a clad modified fiber optic sensor with Ce, Li and Al doped nanocrystalline zinc oxides,” Sensors and Actuators B 156 (2011). Dopants such as CuO have also been employed with metal oxides such as ZrO2 in order to provide sensing through reversible red-ox reactions, however such approaches can suffer from instability under high temperature and/or high reducing agent concentrations. See e.g., Remmel et al., “Investigation on nanocrystalline copper-doped zirconia thin films for optical sensing of carbon monoxide at high temperature,” Sensors and Actuators B 160 (2011).
Weak dynamic range of optical responses of high temperature stable metal oxides to changing gas atmospheres has generally required investigators to amplify the response by applying them to optical fibers with fiber bragg gratings. By periodically modifying the refractive index of the core of the optical fiber, the interaction with a sensing layer can be enhanced by orders of magnitude. However, fiber bragg gratings exhibit an inherent temperature instability above 500° C. regardless of the sensing layer employed and increase the cost and complexity of a sensor device. See e.g. Tang et al., “Acidic ZSM-5 zeolite-coated long period fiber grating for optical sensing of ammonia,” J. Mater. Chem. 21 (2011); see also Jiang et al., “Multilayer fiber optic sensors for in situ gas monitoring in harsh environments,” Sensors and Actuators B 177 (2013); see also Wei et al, “Terbium doped strontium cerate enabled long period fiber gratings for high temperature sensing of hydrogen,” Sensors and Actuators B 152 (2011); see also Remmel et al., “Investigation on nanocrystalline copper-doped zirconia thin films for optical sensing of carbon monoxide at high temperature,” Sensors and Actuators B 160 (2011).
It would be advantageous to provide a method of improving optical responsesof metal oxides to changes in chemical compositions without resort to incorporation of noble metals, such as gold, platinum, and silver and to mitigate the need for advanced sensor designs such as those employing fiber bragg gratings. It would be particularly advantageous if the method of improvement remained effective or even further improved at higher temperatures, in order to avoid the low temperature limitations associated with alternate methodologies. It would be further advantageous if the increased response of the metal oxide material could be brought about by relatively well understood processes, such as doping, and demonstrated reversibility under high temperature conditions of interest.
Presented here is a method of detecting changes in the chemical composition of a gaseous stream by utilizing the optical response of conducting oxide material having a relatively high carrier concentration. The optical response of the conducting metal oxide materials disclosed are believed to stem predominantly from alterations to the carrier concentration that occur within changing gas atmospheres at elevated temperatures. By suitable selection of dopants in conjunction with high temperature stable metal oxides, the surprisingly effective method utilized within this disclosure provides a means whereby conducting metal oxides having relatively high carrier concentrations are employed to generate improved signals under gaseous atmospheres which experience varying concentrations of reducing and oxidizing agents.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.